1. Field of the Invention
The present invention relates to a gantry for use in a magnetic resonance imaging (MRI) apparatus, or a nuclear magnetic resonance (NMR) analyzer.
2. Description of the Related Art
MRI apparatuses and NMR analyzers include a static magnetic field generating section to generate a static magnetic field, which generates a magnetic resonance signal, through quantization of nuclear magnetism. Such a static magnetic field generating section includes a static magnetic field generating unit and a shimming unit. The static magnetic field generating unit generates the static magnetic field. The shimming unit adjusts uniformity of the static magnetic field.
FIG. 7 is a schematic for explaining a static magnetic field generated by a static magnetic field generating unit 10A. The length of the static magnetic field generating unit 10A in the axial direction thereof is relatively longer. The static magnetic field generating unit 10A is formed from a superconducting magnet. Moreover, the static magnetic field generating unit 10A includes a superconducting coil 12A accommodated within a roughly cylindrical vacuum chamber that is filled with a refrigerant. The vacuum chamber has a bore, i.e., a hollow space, in its central portion. A magnetic field (static magnetic field), indicated by dashed lines in FIG. 7, is generated within the bore. Because the static magnetic field is a key factor in an MRI operation and an NMR analysis operation, it is necessary that the static magnetic field is uniform as much as possible. Specifically, it is necessary that the static magnetic field within the imaging area indicated by the dashed-dotted line in FIG. 7 is highly uniform. If the static magnetic field within the imaging area is non-uniform, it is corrected so that the degree of uniformity is less than or equal to several parts-per-million.
A technology called shimming (passive shim) is known in the art as a technology for correcting non-uniformity of the static magnetic field. In the shimming technology, a magnetic field is adjusted by placing several pieces of iron (iron shims) in an appropriate position in the bore. Specifically, a shimming structure actualizing the shimming technology includes, for example, shim trays (not shown) arranged within the bore. A plurality of pockets is formed at different locations on each of the shim trays. An appropriate number of iron shims are placed in an appropriate pocket to make the static magnetic field in the bore uniform. This configuration enables placement of an appropriate number of iron shims in appropriate positions. For details, refer to, for example, JP-A H8-299304 (KOKAI).
FIG. 8 is a schematic for explaining a static magnetic field generated by a static magnetic field generating unit 10B which is seen in recent years and it is relatively short in the axial direction thereof. A trend can be seen in recent years to shorten an axial length of the MRI apparatus to reduce a sensation of being surrounded experienced by a patient (test subject). The static magnetic field generated by the static magnetic field generating unit 10A and 10B composed of the superconducting magnet is significantly disturbed near ends of the static magnetic field generating units 10A and 10B in the axial direction, as shown in FIG. 7 and FIG. 8. Such a disturbance is caused by influence from the superconducting coils 12A and 12B. In the static magnetic field generating unit 10A, which is relatively longer in the axial direction, as shown in FIG. 7, the imaging area is away from positions at which the disturbance occurs. Therefore, the static magnetic field in the imaging area is less affected due to the disturbance so that the non-uniformity of the static magnetic field can be corrected with a small number of iron shims. However, in the static magnetic field generating unit 10B, which is relatively shorter in the axial direction, as shown in FIG. 8, a distance between the superconducting coil 12B and the imaging area indicated by the dashed-dotted line is short. Therefore, the static magnetic field in the imaging area is significantly affected due to the disturbance so that a large number of iron shims are required to achieve uniformity in the static magnetic field.
However, when a large number of iron shims are used as described above, a secondary problem occurs, such as that described below. Magnetic susceptibility of iron changes with temperature. The magnetic susceptibility decreases when the temperature rises. The iron shims are generally disposed near a gradient magnetic coil. Therefore, the iron shims may be heated up due to the temperature generated in the gradient magnetic coil during an MRI operation. When the iron shims are heated in this way, the iron shims do not perform the desired correction so that the uniformity of the static magnetic field deteriorates. As a result, the MRI imaging operation, or the NMR analysis operation, cannot be successfully performed.
Also, a large amount of stress is applied when a large number of iron shims are used because of influence of the magnetic field. As a result, adhered iron shims may come loose. Moreover, for example, the shim trays, in which the iron shims are placed, may become damaged, or the apparatus may become slightly deformed.